Date: Sun, 14 Oct 2012 16:42:23 +0200 From: Jilles Tjoelker <jilles@stack.nl> To: Eitan Adler <lists@eitanadler.com> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: -lpthread vs -pthread: does -D_REENTRANT matter? Message-ID: <20121014144222.GA14503@stack.nl> In-Reply-To: <CAF6rxg=PnQvtXydhx8%2BoRZJ2ERBoGwedXPcGi_9icYxAtPuxVw@mail.gmail.com> References: <CAF6rxg=PnQvtXydhx8%2BoRZJ2ERBoGwedXPcGi_9icYxAtPuxVw@mail.gmail.com>
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On Mon, Oct 08, 2012 at 12:17:08PM -0400, Eitan Adler wrote: > The only difference between -lpthread and -pthread that I could see is > that the latter also sets -D_REENTRANT. > However, I can't find any uses of _REENTRANT anywhere outside of a few > utilities that seem to define it manually. > Testing with various manually written pthread programs resulted in > identical binaries, let alone identical results. > Is there an actual difference between -pthread and -lpthread or is > this just a historical artifact? In some cases, -pthread also affects the compiler's code generation. On some RISC architectures, compilers may try to avoid loads and stores of less than 32 bits. For example (untested): struct { int n; char a, b, c, d; } *p; p->a = p->b = p->c = 0; The compiler might load p->d and then store the 32 bits containing a, b, c and d at once. This causes a race condition if p->d is written concurrently. Because C99 does not specify threading, it allows these transformations. In C11, they are forbidden. Passing -pthread disables them as well. -- Jilles Tjoelker
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